Both petrol and diesel stroke engines typically use a camshaft to control the opening and closing of the engine's intake and exhaust valves. Normally the open period of the valves, usually referred to as the “duration” or “dwell”, is fixed by the valve lobe shape or profile ground onto the lobe of the camshaft when it is manufactured. Normally, this profile cannot be varied without the physical replacement of the camshaft by another with a different profile ground onto its lobes.
On some engines that are described as having variable camshaft timing, the opening and closing points of the valves can be varied but the actual duration or dwell of the valve opening remains fixed. A conventional camshaft that provides a fixed amount of valve opening allows an engine to achieve maximum volumetric efficiency, and hence torque, at only one point in the engine's revolution range. The torque falls off on either side of this point.
A camshaft arrangement which allows the valve opening duration to be varied so as to maximise the torque throughout the engine's revolution range would be very desirable. This fact has long been realised by engine designers and much effort has been expended in the search of a mechanical variable duration system of valve timing. No successful system has been achieved for a mechanical continuously variable system of valve timing duration. Systems which are not continuously variable but operate on a two-stage principle, such as Honda's VTEC system, have been adopted and are highly successful. Much effort is being spent on investigating hydraulic, pneumatic and solenoid systems of variable duration valve timing. Although the main advantage of a variable duration timing camshaft is to improve the torque spread of an engine it could be used to provide throttle-free control of the engine's induction to minimise intake pumping losses and/or to achieve low exhaust emissions.
It has been proposed to use a camshaft having two closely spaced cam lobes in combination with a wider than normal follower, or tappet, that rides on both lobes simultaneously. A mechanism is provided so that the lobes can be aligned to give minimum duration or misaligned to give an increase in duration. If the misalignment does not exceed the angular distance of constant radius of the cam lobe's nose, the follower “sees” the constant radius area as a continuous surface. The main deficiency of these devices is that the useable duration range is limited to twice, measured in degrees of rotation of the crankshaft, that of the angular span of the constant radius at the lobe's nose. The nose is the region of maximum lift of the camshaft lobe. Any attempt to increase the duration past this angular distance results in the follower falling into the gap between the two lobe noses causing unacceptable noise and wear. There have been proposed solutions to this problem, but none have been commercially successful. There is a wide range of possible variations in lobe profiles, style of construction, even using lobes on two separate shafts, and methods of control and actuation of the duration change. However, none of these have provided a successful product.
It would be desirable to have an improved variable duration timing camshaft and even more desirable to have one that could be fitted after market.